At least 40 people were killed in Syrian government air strikes on the rebel-held town of Douma in Eastern Ghouta on Friday, a war monitor reported, in the first air strikes in more than a week in the enclave.

The Syrian Observatory for Human Rights said dozens of air strikes hit various parts of Douma, including some suspected to have been carried out by Russian warplanes.

"At least 40 civilians, including 8 children, have been killed in air strikes and rocket fire on Douma," it said.

State TV said rebels from the Jaish al-Islam group had shelled a crossing point outside Douma and a residential area, triggering a response by the air force.

They were the first bombing raids on Douma since negotiations began around 10 days ago over a withdrawal of rebels and civilians from the town, the largest in Eastern Ghouta.

Backed by Russia, Syrian troops have recaptured 95 percent of Ghouta since 18 February through a combination of a deadly air and ground assault and evacuation deals.

The Syrian government has for the past week been busing Jaish al-Islam fighters out of Douma to rebel-held areas of northern Syria under an "evacuation deal" struck in March.

The largest convoy of evacuees from Eastern Ghouta arrived in Hama last week (AFP)

However, many Jaish al-Islam fighters have refused the terms and promised to continue resistance in the town.

Syria and Russia had threatened a resumption of their brutal assault if Jaish al-Islam did not agree to the deal.

Jaish al-Islam signalled on Friday that it is open to negotiations with Russia to reach a peaceful settlement over Douma.

"We don't want to close the door that can lead to sparing the blood of civilians and reaching a peaceful situation," Jaish al-Islam official Mohammad Alloush said said in an interview with al-Hadath TV. Alloush also denied the group had shelled residential areas of Damascus.

Syria's state news agency SANA said Syrian air strikes hit the town on Friday in response to rebel mortar fire from Douma that wounded seven people near the capital.

The government assault since 18 February has killed more than 1,600 civilians, the Observatory says, and devastated entire neighbourhoods.

The retaking of Douma would mark the complete capture of Eastern Ghouta, a rebel bastion on the outskirts of Damascus, and be a major milestone in President Bashar al-Assad's efforts to regain control of territory seized by rebel factions during Syria's seven-year civil war.

Backed by Russia, Government forces have scored a series of victories over rebel forces in recent years, often through campaigns of siege, aerial bombardment and ground offensives that have drawn widespread international condemnation.

Syria's conflict has killed more than 350,000 people since 2011 and spiralled into a complex war involving world powers.